1. Field of the Invention
This invention relates to improvements in energy saving devices and methods for use in polyphase electrical systems, and more particularly to improvements in methods and devices for controlling the power delivered to a polyphase motor in a system including variable mechanical load, such as a gas/fluid compressor of the type used in air conditioning systems, or the like.
2. Description of the Related Art
In the past, many energy saving devices have been proposed for use in electrical systems which include a motor. Many popular commercial power saver devices are used to save energy in home appliances, such as refrigerators or freezers in which an electric motor is operated with electrical power by conditioning the power from that supplied by a standard commercial power supply.
Typically, such devices rely upon one or more voltage waveform "voltage chopper," of the type in which portions of a voltage waveform are extinguished during certain portions of a voltage cycle. In the context of an ac voltage waveform, this results in the voltage waveform existing only during portions of each individual alternating cycle, with the voltage value being held at zero during the remaining cycle portions.
Although many such energy saving systems have been proposed for single phase motors, in the context of multi-phase motor systems, to which the present invention pertains, most single phase units cannot feasiblely be used. Most such single phase units achieve their power saving functions by constantly correcting the phase between the supplied voltage and current, thereby increasing the overall power factor to make the system more efficient. This technique is not simply applied in the implementation of such power saving devices used for multi-phase systems.
One of the problems of such multi-phase systems that is often encountered, however, which makes the application of energy saving devices difficult, is that the mechanical loads on the driving motor may vary, and such load variations are generally not considered in providing an appropriate level of power to the motor. For example, in an air conditioning compressor of the type that compresses a hot gas, such as gaseous Freon, to exchange the heat therein during the formation of a fluid Freon, initially at start-up, a much larger power is required than is required after the compressor reaches its steady-state operation. Nevertheless, energy saving devices generally still apply a constant voltage to the motor.
In addition, on startup, multi-phase motors usually include start-up windings that are initially energized, but which are removed from the circuit once the motor has reached its normal running speed. Thus, at normal running speeds, a lesser amount of power may be sufficient to sustain the operation of the motor. Nevertheless, many energy saving devices and techniques apply full power to the motor during startup, and continue to provide full power to the motor after it has reached normal running speed. Continuing to apply full power to the motor under such circumstances is wasteful and unnecessary.
Moreover, in prior devices and techniques in which constantly changing voltage, current, or power factor control is provided, a hysteresis effect occurs due to the demands of the load. This reduces the potential efficiency savings that might otherwise be achieved. Thus, although such power savings devices and techniques may be useful in systems in which the load on the motor is relatively constant, they are not well suited for achieving energy savings in systems in which the load on the motor varies, such as, again, for example, in air conditioner compressors, or the like.
What is needed, therefore, is an energy conserving device and technique that can be used in conjunction with multi-phase motors that may have varying power needs used to drive loads which may provide varying loads to the motor.
Those concerned with these and other problems recognize the need for an improved energy conservation circuit and method for use with multi-phase motors used to power a variable load, or the like.